The present invention relates to the field of electronic solid state ballasts for High Intensity Discharge (HID) lamps, and more specifically, it relates to a method and device utilizing solid state ballasts for operating HID lamps, e.g., High Pressure Sodium (HPS) lamps.
The term xe2x80x9cdischarge lampxe2x80x9d refers to a lamp in which the electric energy is transformed into optical radiation energy when electric current is passed through a gas, metal vapor, or a mixture thereof, present inside the lamp.
Presently, various circuits of electronic ballasts for discharge lamps, and in particular for fluorescent lamps, are known in the art. A specific example is the circuit shown in FIG. 1, which uses two power switches PS1 and PS2 in a totem pole (half-bridge) topology, the tube circuit consisting of an L-C series resonant circuit. The power switches represented by power MOSFETS are driven to alternatively conduct, e.g., by a MOS Gate Driver (IR2155)(MGD). The MGD provides a high frequency (20 to 80 kHz) square wave output, with the frequency of oscillation given by:       f    osc    =            1              1.4        ⁢                  R          T                ⁢                  C          T                      .  
Prior to striking the fluorescent lamp 2, the resonant circuit consists of L, C1 and C2 connected in series. Since C2 has a lower value than C1, it operates at a higher AC voltage than the latter, and in fact, it is this higher voltage that strikes the lamp. After the lamp strikes, C2 is effectively shorted by the lamp voltage drop, and the resonant frequency of the circuit is now determined by L and C1.
Under resonance conditions, the sinusoidal voltage across the lamp is amplified by a factor of Q (Q being the circuit quality factor) and the amplitude of this voltage attains a value sufficient for striking the lamp, which thereafter gives a non-blinking light.
The above-described basic circuit is well-suited for fluorescent lamps, but will not adequately work for arc discharge lamps or HID lamps.
Initially, the HID lamp is an open circuit. Short pulses of voltage suffice to strike the lamp, provided the pulses are of adequate amplitude (about 4,500 Volts). Subsequent to striking, the resistance of the lamp drops drastically and then slowly rises to its normal operating level. Hence, to prevent lamp damage subsequent to striking and during the warm-up, the current of the lamp must be restricted.
It is a characteristic of HID lamps that their voltage increases over the life of the lamp, due to a slow increase of stabilization temperature. Therefore, unless the lamp ballast maintains the lamp power, the light output of the lamp will vary to an unacceptable degree.
Ballast devices for HD lamps should be different from ballasts for fluorescent lamps, for the following main reasons:
1) these devices should withstand open-circuit operation conditions;
2) they should supply sufficiently high power for striking the lamp at a voltage of 3 to 4 kV;
3) they should adapt themselves to large variations of the lamp voltages;
4) the ballasts should not destabilize the lamp arc discharge, and
5) the ballasts should be compatible with lamp characteristics, so as to maximize the lamp""s service life.
Therefore, when replacing the fluorescent lamp of FIG. 1 with an HID lamp 4, as shown in FIG. 2, the ballast of FIG. 1 will not operate the HD lamp, for the following major reasons:
An HID lamp is not consistently susceptible to striking and is not necessarily in a state of readiness for striking. In fact, the circuit of FIG. 1 enables a low power (70-150 W), cold HID lamp to be struck and even brought to the operation mode. But if the lamp has operated at rated power and is shut off for some reason, the subsequent attempt to switch on the hot lamp will prove to be unsuccessful and will damage the main components of the circuit, first of all, the power switches.
As can be seen in FIG. 2, the oscillation circuit is shorted only when the lamp is struck (the lamp shortens the C2 capacitor). In all other situations, when the lamp is not struck; the lamp is not present; the lamp is damaged; the lamp circuit is broken, etc., the oscillation circuit is not shortened, which inevitably results in a failure of the device.
Therefore, the direct use of an electronic ballast intended for fluorescent lamps in HID lamp circuits is ruled out, since it is impossible for such a ballast to provide reliable operation of an HID lamp under actual operating conditions.
It is thus a broad object of the present invention to provide a method for operating HID lamps with devices built according to the basic topology of electronic ballasts for fluorescent lamps, which takes into account significant physical and design features of these lamps, such as their insusceptibility to striking and the fact that in the absence of a lamp in the circuit, the series L-C circuit is not broken. The method thus provides optimal conditions for striking, heating and operation of HID lamps.
The invention provides a method for operating electronic ballasts for High Intensity Discharge (HID) lamps, said electronic ballasts having a driver, two power switches connected in a half-bridge arrangement, an LC series circuit, a driver controller for controlling the operation of the driver, a current sensor in the lamp circuit, and a power sensor in the power switch circuit, said method comprising (a) generating pulses of frequency f1 for a duration of time t1 being equal to n/f1, where n is a positive number, and f1 equals the resonance frequency of the ballast""s LC series circuit; (b) monitoring the existence of current in the lamp circuit after the duration of time t1 has elapsed, and in the event that there is no current in the lamp circuit, proceeding to step (h); (c) monitoring the current in the lamp circuit, and proceeding to step (h) upon determining that the current in the lamp circuit has ceased to flow; (d) continuing the generation of said pulses of frequency f1 for a predetermined duration of time t2 counting from the start of the generation of said pulses according to step (a); (e) switching the frequency f1 of said pulses to an operating frequency f2, at which a set power for the lamp is reached; (f) monitoring the power on the lamp and stabilizing this power at the level of the power set for the lamp, by gradually modifying the frequency f2, and proceeding to step (h) in the event that the power in the lamp circuit exceeds the power set for the lamp by a given margin; (g) monitoring the current in, and power of, the lamp circuit according to steps (c) and (f); (h) inhibiting the generation of said pulses for a predetermined duration of time exceeding t1 and approximately equal to t2/k, where k is a positive number; (i) proceeding to step (a) until the said predetermined duration of time t2, counted from the start of the generation of pulses according to step (a), has elapsed; and (j) inhibiting the generation of said pulses until the power to the ballast is first switched off and then on.
In accordance with the invention, there is also provided a device for operating electronic ballasts for High Intensity Discharge (HID) lamps, said electronic ballasts having a driver, a power switching circuit including two power switches connected in a half-bridge arrangement, and an LC series circuit, said device comprising a driver controller for controlling the operation of said driver, a current sensor connected on a line leading and adjacent to an electrode of the HID lamp, and a power sensor incorporated in the power switching circuit.